Method for producing high purity aluminum

ABSTRACT

The present invention provides a method for producing high purity aluminum by immersing a cooling body in molten aluminum, and forming and growing crystals of the high purity aluminum on the cooling body by rotating the molten aluminum around the cooling body and introducing bubbles of a gas into the rotating molten aluminum.

FIELD OF THE INVENTION

The present invention relates to a method for producing high purityaluminum. More particularly, the present invention relates to a methodfor producing high purity aluminum by crystallizing the same from moltenaluminum containing eutectic crystal impurities such as Si and Fe, bythe use of the segregation principle.

BACKGROUND OF THE INVENTION

Conventionally, as to methods for producing high purity aluminumaccording to the segregation principle, there has been known: a methodin which the molten aluminum in the vicinity of a solidificationinterface is stirred such that when molten aluminum is cooled, it issolidified from a downward or circumferential direction; a method inwhich a cooling body is immersed in the molten aluminum, and the coolingbody is rotated while supplying a cooling medium into the cooling body,thereby crystallizing purified aluminum with high purity on thecircumferential surface; and the like.

In the purification method according to the segregation principle, theconcentrated layer of impurities discharged into the molten aluminum inthe vicinity of the solidification interface is reduced in thickness asmuch as possible to disperse the impurities throughout the moltenaluminum. This results in an improvement of purification efficiency. Inorder to improve purification efficiency as described above at arelatively fast production velocity, there has been proposed a techniquewhereby the relative velocity between crystallized aluminum and moltenaluminum is increased to enhance the efficiency of dischargingconcentrated impurities into the molten aluminum having less impurities.For example, there has been disclosed a method in Japanese PatentPublication No. 61-3385, wherein a cooling body is rotated so that therelative velocity between the outer regions of the cooling body and themolten aluminum falls in the range of 1600 mm/s to 8000 mm/s, wherebythe concentrated layer of impurities in the vicinity of solidificationinterface is reduced in thickness to enhance the purity of the obtainedaluminum. Also, in Japanese Patent Publication No. 63-64504, there hasbeen proposed the following method: in solidifying molten aluminum froma downward direction, bubbles are released from the center of the lowerpart of a rotating body into the molten aluminum in the vicinity of thesolidification interface to induce a concentrated layer of impurities todisperse. This causes the impurities to disperse throughout the moltenaluminum, thereby enhancing the purity of the purified aluminum.

Further, as a method for removing hydrogen from molten aluminum withefficiency, there has been proposed a method in Japanese PatentPublication No. 5-852558, in which molten aluminum is stirred, and a gaswhich can remove hydrogen is blown into the molten aluminum to solidifythe molten aluminum in one direction.

However, with these conventional methods, impurities in the obtainedaluminum cannot be removed to a sufficient degree. For example, in themethod of using a rotated cooling body, it is desirable to make therelative velocity between the outer regions of the cooling body and themolten aluminum as high as possible in order to enhance the purity ofthe obtained aluminum. However, since the molten aluminum also flows inthe same direction in accordance with the rotation of the cooling body,there is a limit on the effect of reducing the concentrated layer ofimpurities in thickness. Also, even if bubbles are discharged from thecenter of the lower part of the rotating body into the liquid phase inthe vicinity of the solidification interface when the molten aluminum issolidified from a downward direction, the bubbles are lighter in weightthan the molten aluminum, and hence move upwards. This limits thearrival at the concentrated layer of impurities in the vicinity of thesolidification interface, and the stirring function thereof. Further,even if the molten aluminum is stirred and a gas which can removehydrogen is blown into the molten aluminum, during which theunidirectional solidification of the molten aluminum is effected, andthe hydrogen is removed from the molten aluminum with high efficiency,the removal efficiency of the eutectic crystal impurities such as Si andFe is not necessarily sufficient.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forproducing high purity aluminum, which has overcome the forgoingproblems.

The present inventors have intensively studied on the purification ofaluminum according to the segregation principle, and have discovered ahighly advantageous method for producing high purity aluminum withextremely high purification efficiency by using a cooling body, and havethus completed the present invention.

That is, the present invention provides a method for producing a highpurity aluminum, said method comprising the steps of:

immersing a cooling body in molten aluminum, wherein the molten aluminumis at a temperature that is within a liquid phase temperature rangethereof, and wherein the surface temperature of an exterior surface ofthe cooling body is less than said liquid phase temperature range of themolten aluminum; and

forming and growing crystals of the high purity aluminum on the exteriorsurface of the cooling body by (i) rotating the molten aluminum aroundthe cooling body such that the centrifugal acceleration exerted on themolten alualuminum said rotation falls within the range of 0.01 m/s² to1500 m/s², and (ii) introducing bubbles of a gas into the moltenaluminum such that the amount of gas bubbles introduced into the moltenaluminum falls within the range of from 0.01 to 150 liters, whenmeasured at 25° C. and atmosphere of pressure(760 mm Hg), for eachkilogram of the high purity aluminum that is forming and growing ascrystals on the surface of the cooling body.

DETAILED DESCRIPTION OF THE INVENTION

With the method of the present invention, molten aluminum is rotatedaround a cooling body, and the reaction force to the centrifugal forceexerted on the molten aluminum makes bubbles of an introduced gas movetoward a solidification interface between purified aluminum that iscrystallized on the surface of the cooling body and the molten aluminum.Then, the bubbles chafe against the solidification interface and in thevicinity thereof with surfacing. Consequently, a concentrated layer ofimpurities is removed with high efficiency by the bubbles of theintroduced gas at the solidification interface, thereby extremelyenhancing the purification efficiency of the inventive method, wherebypurified aluminum is obtained. That is, in accordance with the method ofthe present invention, the reaction force to the centrifugal forceexerted on the molten aluminum can be used to cause the bubbles of theintroduced gas in the molten aluminum to migrate to the solidificationinterface with efficiency, and makes the gas bubbles rise with chafingagainst the solidification interface. This enables the removal of theconcentrated layer of the impurities arising on the solidificationinterface with high efficiency.

In the present invention, as a method for rotating molten aluminum,there are included methods of using a viscous flow effect resulting fromthe rotation of a vessel for holding the molten aluminum, animpeller-like stirrer, a rotating magnetic field, and the like, but anymethod can be available for rotating the molten aluminum. The coolingbody may also be rotated in combination with other methods of rotatingthe molten aluminum, if so desired.

In the method of the present invention, the molten aluminum is rotatedaround the cooling body so that the centrifugal acceleration exerted onthe molten aluminum falls in the range of 0.01 m/s² to 1500 m/s²preferably in the range of 0.1 m/s² to 800 m/s², more preferably in therange of 1 m/s² to 500 m/s². When the centrifugal acceleration exertedon the molten aluminum is less than 0.01 m/s², the bubbles of theintroduced gas insufficiently reach the solidification interface betweenthe purified aluminum crystallized on the surface of the cooling bodyand the molten aluminum, and the vicinity thereof. When it exceeds 1500m/s², it is difficult for the gas bubbles to come up to the surface andleave, due to the large centrifugal force. This results in the formationof a cavity on the surface of the cooling body, which makes it difficultfor aluminum to crystallize and grow.

In the method of the present invention, the gas bubbles can beintroduced into the molten aluminum with through various methodsincluding: a method of using a separate gas bubble introduction tubefrom the cooling body; a method in which the bottom of the holdingvessel is provided with a minute gas bubble introduction port; a methodin which introduction is conducted through an opening at the bottom ofthe cooling body; and the like. It is preferable that the gas bubbleintroduction port is established so as to introduce gas bubbles into themolten aluminum rotating around the cooling body so that the centrifugalacceleration exerted on the molten aluminum falls in the range of 0.01m/s² to 1500 m/s².

In the present invention, the preferable distance range in which themolten aluminum is rotated around the cooling body differs dependingupon the method for rotating the molten aluminum. However, it is enoughif the gas bubbles can be introduced so that the gas bubbles introducedinto the molten aluminum can reach the solidification interfaceefficiently and move upwards with chafing against the solidificationinterface by using the reaction force to the centrifugal force exertedon the molten aluminum, to thereby efficiently remove the concentratedlayer of impurities arising on the solidification interface. It ispreferable that the gas bubbles occupy a distance between thesolidification interface (aluminum crystallized on the surface of thecooling body) and the molten aluminum which falls in the range of 1 mmto 10 mm, in the vicinity of the cooling body.

In order to further improve the purification efficiency, it is morepreferable that the mass of molten aluminum being rotated cover as widerange as possible. That is, it is more preferable that the moltenaluminum, from the solidification interface to the internal wall ofholding vessel of the molten aluminum, rotates at the rotating velocityof the present invention.

In the present invention, as the kind of gas for forming the gas bubblesto be introduced into the molten aluminum, any gas or gases may beemployed, so long as the gas (or gases) is in the gas state at thetemperature of the molten aluminum. However, gases which will notdissolve in a large amount in the molten aluminum are preferable. Thus,an inactive gases to molten aluminum (such as helium gas, argon gas),substantially inactive gas to the molten aluminum (such as nitrogengas), air, chlorine gas, chloride gas, and mixed gases thereof areavailable. As chloride gas, a volatile flux such as zinc chloride,aluminum chloride, titanium (IV) chloride, hexachloroethane, carbontetrachloride, and hexachlorobenzene, that are in the gas state in themolten aluminum are also available.

Especially, air forms a tough film of oxide on the surface of thebubble, and hence it has a high effect of chafing the solidificationinterface and the vicinity thereof, thereby removing the concentratedlayer of the impurities at the solidification interface with higherefficiency. Consequently, the purification efficiency of aluminum can befurther improved. Moreover, air is available at lower cost than in thecase of other gases, and hence it is preferable.

Also, the dew point of each gas described above is not specificallylimited. However, in the cases of the inactive gas and substantiallyinactive gas, it is preferable that steam is contained therein, and thedew point is preferably in the range of -8° C. to 30° C. When the dewpoint is -8° C. or more, the purification efficiency may becomeextremely high, thus being preferable. When the dew point exceeds 30°C., dew condensation may tend to occur in the piping and on theperiphery of the apparatus. Accordingly, if moisture attached due to dewcondensation drops in the molten aluminum, an explosion may likelyoccur.

It is preferable that not too much moisture be contained in each of thegases described above, so as to help avoid the possibility of anexplosion that might otherwise occur as said moisture rapidly changes tosteam in the rotating molten aluminum.

In the present invention, the amount of gas for forming the gas bubblesto be introduced into the molten aluminum may differ depending upon thekind of gas utilized. However, generally, it is preferable that theamount of gas added is in the range of 0.01 to 150 liters (25° C., 1 atmpressure(760 mm Hg)), preferably 0.1 to 100 liters (25° C., 1 atmpressure(760 mm Hg)), per kg of aluminum to be purified and recovered.When the amount of gas to be introduced is less than 0.01 liter per kgof aluminum to be purified and recovered, the purification effectachieved is small. On the other hand, when it is greater than 150 litersor more, its introduction velocity becomes larger than the surfacingvelocity of the gas bubbles, causing vigorous scattering of the moltenaluminum on the molten metal surface, and also resulting in large metalloss. The more preferable amount of gas to be introduced is in the rangeof 0.1 to 100 liters per kg of aluminum to be purified.

In the molten aluminum rotating around the cooling body, a centrifugalforce arises due to the rotational movement of the molten aluminumitself. The magnitude of the centrifugal force is expressed by thefollowing equation, where a represents the centrifugal acceleration.##EQU1##

In the above equation, r represents the distance from the center of therotational movement, ω represents the angular velocity of the moltenaluminum, and ν represents the rotational velocity of the moltenaluminum. The gas bubbles introduced into the molten aluminum inrotational movement are accelerated towards the center of the rotationalmovement (i.e., the solidification interface between the purifiedaluminum crystallized on the surface of the cooling body and the moltenaluminum) by the reaction force to the centrifugal force expressed bythe above equation, so that the bubbles reach and are pressed againstthe solidification interface. When the gas bubbles reach thesolidification interface, they also act on a concentrated layer ofimpurities arising in the vicinity of the solidification interface.Specifically, an intense stirring effect occurs on the concentratedlayer of the impurities, as the reaction force to the centrifugal forcecauses the gas bubbles to go upward while chafing against thesolidification interface, which removes the concentrated layer ofimpurities and enhances a dispersion of the impurities from theconcentrated layer of impurities into the molten aluminum (whichcontains a lower impurity concentration than the concentrated layer ofimpurities).

For example, when the molten aluminum is rotated by a viscous floweffect arising from the rotation of the vessel for holding the moltenaluminum, and if the flow is a laminar flow, the rotating velocity u ofthe molten aluminum and the centrifugal acceleration a are expressed bythe following equations, respectively.

    u=r.sup.2 (r.sup.2 -r.sub.1.sup.2)/r(r.sub.2.sup.2 -r.sub.1.sup.2)

    a=u.sup.2 /r

where r represents a distance from the center of the cooling body, r₁represents the radius of the cooling body, r₂ represents the internalradius of the holding vessel, and represents the angular velocity of theholding vessel.

The aforementioned centrifugal acceleration can be derived from thedistance from the center of the rotational movement and the rotationalvelocity of the molten aluminum. Generally, it is extremely difficult todirectly measure the rotational velocity of the molten aluminum. Thus,Reynolds' principle of similarity is user to estimate the rotationalvelocity of the molten aluminum. According to Reynolds' principle ofsimilarity, even if mutual systems differ in the scale of representativelength L, the scale of velocity U, the density of fluid, and thecoefficient of viscosity, if both Reynolds numbers are the same, theflow fields are dynamically analogous. The Re is expressed by thefollowing equation: ##EQU2## where U represents the representativevelocity, L represents the representative length, and ν represents thecoefficient of kinematic viscosity (μ/ρ). Concretely, a rotationalvelocity measuring setup is prepared, wherein the scale, and theconditions on which the fluid is rotated are the same as those in thepurification apparatus, and a fluid having the same coefficient ofkinematic viscosity as that of the molten aluminum is used so that the"Re" values become equal. If a transparent fluid (model fluid) havingthe same coefficient of kinematic viscosity as that of the moltenaluminum at temperatures around room temperature is used, the flow fieldof the molten aluminum can be recreated at room temperature, so that theobservation thereof becomes possible. Resin powder particles (i.e., atracer) with substantially the same density as that of the fluid aremixed and dispersed in the model fluid to determine the velocity of theparticles, thereby enabling an estimation of the rotational velocity ofthe molten aluminum.

Accordingly, when utilizing the procedure described above, thecentrifugal acceleration exerted on the molten aluminum is preferablymeasured according to a tracer method by using polyether sulfone as atracer and ethanol as a model fluid, taking measurements at an analogousdistance of 1 mm to 10 mm from the solidification interface.

The purified aluminum obtained by the method of the present inventioncan be used as a material for aluminum foil for an electrolyticcapacitor. The purified aluminum obtained according to the method of thepresent invention, when used as a material for aluminum foil for anelectrolytic capacitor, has preferably a Si content of 35 wt ppm orless, and a Fe content of 30 wt ppm or less, and more preferably a Sicontent of 30 wt ppm or less, and a Fe content of 20 wt ppm or less.

In the method of the present invention, the content of Si and Fe in thepurified aluminum varies depending upon the centrifugal accelerationexerted on the molten aluminum rotating around the cooling body.Therefore, it is possible to select the centrifugal acceleration,rotating velocity, and method for causing the rotation corresponding tothe required purity of the purified aluminum.

The purified aluminum obtainable by the method of the present inventioncan be subjected to subsequent manufacture steps such as slab casting,hot rolling, cold rolling, and foil rolling to be processed intoaluminum foil for an electrolytic capacitor as described, for example,in "Foundation and Industrial Technology of Aluminum Materials"(corporation Japan Light Metal Association), pp. 347 to 350.

Also, the obtained high purity aluminum is preferably used to preparefoil for an electrolytic capacitor, a sputtering target, a substrate fora hard disk, a superconducting stabilizing material, a bonding wire, andthe like.

The present invention will be described in details by way of theexamples, which should not be construed as limiting the scope of theinvention.

EXAMPLE 1

Molten aluminum containing Si of 50 ppm and Fe of 69 ppm as impuritiesis put into a crucible made of graphite with an inner diameter of 100 mmand is heated and kept at 665° C. by a heater. Then, argon gas isintroduced at a flow rate of 0.5 l/min from the center of the bottom ofthe crucible, and a cooling body made of graphite with an externaldiameter of 30 mm is immersed in the molten aluminum. Then, the crucibleis rotated at a rotation speed of 60 rpm while supplying nitrogen gasfor cooling into the cooling body. At this step, the relative velocitybetween the outer regions of the cooling body and the molten aluminum is94 mm/s. As a result, the flow is a laminar flow, and the rotatingvelocity of the molten aluminum located at a distance of 1 mm from thesolidification interface towards the molten aluminum is 0.013 m/s, whilethe centrifugal acceleration at this position is 0.011 m/s². Therotating velocity of the molten aluminum located at a distance of 10 mmfrom the solidification interface towards the molten aluminum is 0.11m/s, while the centrifugal acceleration at this position is 0.49 m/s².These values show almost no changes at the completion ofcrystallization. The amount of argon gas introduced is 89 liters per kgof aluminum recovered, while the solidification rate is 50 mm/h. In thismanner, the rotation of the crucible is stopped after about 45 g ofaluminum is crystallized, and the cooling body is pulled up to recoverthe crystallized aluminum. The impurity concentration of the recoveredaluminum is 8 ppm for Si, and 3 ppm for Fe. The purification coefficient(concentration in a purified aluminum/concentration in a raw material)are 0.16 for Si and 0.043 for Fe.

EXAMPLE 2

Molten aluminum containing Si of 200 ppm and Fe of 370 ppm as impuritiesis put into a crucible made of graphite with an inner diameter of 100 mmand is heated and kept at 665° C. by a heater. Then, argon gas isintroduced at a flow rate of 0.5 l/min from the center of the bottom ofthe crucible, and a cooling body made of graphite with an externaldiameter of 30 mm is immersed in the molten aluminum. Then, the crucibleis rotated at a rotation speed of 60 rpm while supplying nitrogen gasfor cooling into the cooling body. At this step, the relative velocitybetween the outer regions of the cooling body and the molten aluminum is94 mm/s. As a result, the flow is a laminar flow, and the rotatingvelocity of the molten aluminum located at a distance of 1 mm from thesolidification interface towards the molten aluminum is 0.013 m/s, whilethe centrifugal acceleration at this position is 0.011 m/s². Therotating velocity of the molten aluminum located at a distance of 10 mmfrom the solidification interface towards the molten aluminum is 0.11m/s, while the centrifugal acceleration at this position is 0.49 m/s².These values show almost no changes at the completion ofcrystallization. The amount of argon gas introduced is 50 liters per kgof aluminum recovered, while the solidification rate is 36 mm/h. In thismanner, the rotation of the crucible is stopped after about 100 g ofaluminum is crystallized, and the cooling body is pulled up to recoverthe crystallized aluminum. The impurity concentration of the recoveredaluminum is 68 ppm for Si, and 86 ppm for Fe. The purificationcoefficient are 0.34 for Si and 0.23 for Fe.

Comparative Example 1

An experiment is carried out on the same conditions as those in example2, except that the crucible is not rotated (relative velocity is 0mm/s). The rotating velocity of the molten aluminum at this step isroughly 0 m/s at any positions, and the centrifugal acceleration at thisposition is 0 m/s² . The amount of argon gas introduced is 36 liters perkg of aluminum recovered, while the solidification rate is 38 mm/h. Theimpurity concentration of about 220 g of the obtained aluminum is 85 ppmfor Si, and 145 ppm for Fe. The purification coefficient are 0.43 for Siand 0.39 for Fe.

Comparative Example 2

An experiment is carried out on the same conditions as those in example2, except that argon gas is not introduced into the molten aluminum.

At this step, the relative velocity between the outer regions of thecooling body and the molten aluminum is 94 mm/s. As a result, the flowis a laminar flow, and the rotating velocity of the molten aluminumlocated at a distance of 1 mm from the solidification interface towardsthe molten aluminum is 0.013 m/s, while the centrifugal acceleration atthis position is 0.011 m/s². The rotating velocity of the moltenaluminum located at a distance of 10 mm from the solidificationinterface towards the molten aluminum is 0.11 m/s, and the centrifugalacceleration at this position is 0.49 m/s². These values show almost nochanges at the completion of crystallization. The rotating velocity ofthe molten aluminum in contact with the solidification interface is 0m/s, and the centrifugal acceleration at this position is 0 m/s². Thesolidification rate is 30 mm/h. The impurity concentration in 140 g ofthe obtained aluminum is 79 ppm for Si, and 110 ppm for Fe. Thepurification coefficient are 0.40 for Si and 0.30 for Fe.

The above results are shown in the following table.

                                      TABLE 1                                     __________________________________________________________________________                             Centrifugal             Impurities in                           Introduced gas                                                                              acceleration    Impurities in                                                                         purified                                    Flow                                                                              Amount of                                                                           (110 mm from                                                                           Solidification                                                                       raw material                                                                          aluminum                                                                              Purification                    Kind                                                                              rate                                                                              introduction                                                                        solidification                                                                         rate   Si  Fe  Si  Fe  Coefficient          Crucible   of gas                                                                            (l/min)                                                                           (l/kg Al)                                                                           interface) (m/s.sup.2)                                                                 (mm/h) (ppm)                                                                             (ppm)                                                                             (ppm)                                                                             (ppm)                                                                             Si Fe                __________________________________________________________________________    Example 1                                                                           Rotary                                                                             Argon                                                                             0.5 89    0.011-0.49                                                                             50      50  69  8   3  0.16                                                                             0.043             Example 2                                                                           Rotary                                                                             Argon                                                                             0.5 50    0.011-0.49                                                                             36     200 370 68   86 0.34                                                                             0.23              Comparative                                                                         Stationary                                                                         Argon                                                                             0.5 36    0        38     200 370 85  145 0.43                                                                             0.39              example 1                                                                     Comparative                                                                         Rotary                                                                             None                                                                              0    0    0.011-0.49                                                                             30     200 370 79  110 0.40                                                                             0.30              example 2                                                                     __________________________________________________________________________

Apparent from the results shown in the above table, according to thepresent invention, the introduced gas bubbles act on the solidificationinterface, thereby enabling the removal of eutectic crystal impuritiesin aluminum with high efficiency. Contrary to this, as in thecomparative examples, even if the gas bubbles are introduced into themolten aluminum, if the molten aluminum is not rotated around thecooling body, the eutectic crystal impurities in aluminum cannot beremoved sufficiently. Also, even if the molten aluminum is rotatedaround the cooling body, if the gas bubbles are not introduced, theeutectic crystal impurities in aluminum cannot be removed sufficiently.

What is claimed is:
 1. A method for producing a high purity aluminum,said method comprising the steps of:immersing a cooling body in moltenaluminum containing eutectic impurities, wherein the molten aluminum isat a temperature that is within a liquid phase temperature rangethereof, and wherein the surface temperature of an exterior surface ofthe cooling body is less than said liquid phase temperature range of themolten aluminum; and forming and growing crystals of the high purityaluminum on the exterior surface of the cooling body by (i) rotating themolten aluminum around the cooling body such that the centrifugalacceleration exerted on the molten aluminum by said rotation fallswithin the range of 0.01 m/s² to 1500 m/s², and (ii) introducing bubblesof a gas into the molten aluminum such that the amount of gas bubblesintroduced into the molten aluminum falls within the range of from 0.01to 150 liters, when measured at 25° C. and 1 atmosphere of pressure (760mm Hg), for each kilogram of the high purity aluminum that is formingand growing as crystals on the surface of the cooling body.
 2. Themethod for producing a high purity aluminum according to claim 1,wherein the molten aluminum is rotated around the cooling body by arotating magnetic field, a stirring member, rotating a vessel that holdsthe molten aluminum, or a combination thereof.
 3. The method forproducing a high purity aluminum according to claim 1, wherein the gasfor forming the gas bubbles is selected from the group consisting of agas inactive to molten aluminum, nitrogen, air, chlorine gas, chloridegas, and mixtures thereof.
 4. The method for producing a high purityaluminum according to claim 2, wherein the gas for forming the gasbubbles is selected from the group consisting of a gas inactive toaluminum, nitrogen, air, chlorine gas, chloride gas, and mixturesthereof.
 5. The method for producing a high purity aluminum according toclaim 1, wherein the gas that is forming the gas bubbles is air.
 6. Themethod for producing a high purity aluminum according to claim 2,wherein the gas that is forming the gas bubbles is air.
 7. The methodaccording to claim 1, wherein said eutectic impurities are Si and/or Fe.8. The method according to claim 1, wherein said high purity aluminumhas a Si content of 35 wt. ppm or less.
 9. The method according to claim1, wherein said high purity aluminum has a Fe content of 30 wt. ppm orless.
 10. The method according to claim 3, wherein said inactive gas ishelium or argon.
 11. The method according to claim 4, wherein saidinactive gas is helium or argon.
 12. A method for producing a highpurity aluminum, said method comprising the steps of:immersing a coolingbody in molten aluminum containing eutectic impurities, wherein themolten aluminum is at a temperature that is within a liquid phasetemperature range thereof, and wherein the surface temperature of anexterior surface of the cooling body is less than said liquid phasetemperature range of the molten aluminum; and forming and growingcrystals of the high purity aluminum on the exterior surface of thecooling body by (i) rotating the molten aluminum around the cooling bodysuch that the centrifugal acceleration exerted on the molten aluminum bysaid rotation falls within the range of 0.01 m/s² to 1500 m/s² whenmeasured according to a tracer method using polyether sulfone as atracer and ethanol as a model fluid, and (ii) introducing bubbles of agas into the molten aluminum such that the amount of gas bubblesintroduced into the molten aluminum falls within the range of from 0.01to 150 liters, when measured at 25° C. and 1 atmosphere of pressure (760mm Hg), for each kilogram of the high purity aluminum that is formingand growing as crystals on the surface of the cooling body.
 13. Themethod according to claim 12, wherein said eutectic impurities are Siand/or Fe.
 14. The method according to claim 12, wherein said highpurity aluminum has a Si content of 35 wt. ppm or less.
 15. The methodaccording to claim 12, wherein said high purity aluminum has a Fecontent of 30 wt. ppm or less.